The present invention is related to a radiological tomographic imaging apparatus such as an X-ray CT (computed tomography).
The X-ray CT includes a beam emitter system and detector system, i.e., X-ray emitter system and detector system, which are opposedly placed at both sides of a subject, and are rotated about the subject while emitting an X-ray beam to the subject (scanning). The X-ray beam transmitted through the subject is detected by the detector system around the subject in a plurality of view directions to measure the data projected through the subject by the X-ray beam in each direction. A tomographic image of the subject can be reconstructed based on the projected data.
To obtain a tomographic image, the projection data from the subject may be preprocessed, and thus preprocessed projection data may be used to reconstruct an image according to for example a filtered back projection method.
In the filtered back projection method, projection data of the subject will be filtered as needed for image reconstruction and the filtered projection data will be back projected to reconstruct an image.
In the filtering, a variety of filters can be selected according to the diagnostic purpose and the subject lesion. The filtering consists of FFT (fast Fourier transfer) of the data, multiplication by the selected filter with the FFT data, and IFFT (inverse FFT). In addition, high frequency noises in the projection data may be eliminated during the filtering. The noise reduction technique on high frequency noises included in the projection data can be seen for example in Patent Document 1:
[Patent Document 1]
JP-A-2000-83946
When there exists a foreign matter that is different from the components of a human body, such as metals, in the subject, the value in the detected projection data in the part corresponding to the foreign matter may abruptly change, to become a part noncontinuous from other parts. This is because the X-ray absorption rate of metals is very high.
In case in which projection data including the image of foreign matter such as a metal is filtered, data value of the part including the foreign matter may contain high frequency components, which value may abruptly change. The data value in this part may become changed so that the projection data after the inverse FFT may have some distortion such as undershoot and overshoot.
There is a disadvantage that if the projection data having such distortion is used to back projection to reconstruct an image, streak artifact may be occurred in the image part corresponding to the foreign matter such as metals.